1) Remove prefix from Identifier value as requested by the authors of the proposal.
2) Fixed IdentifierElement => IdentifierType so consistent with example and other schema types.
3) Added an annotation to IdentifierType to describe usage:
<xs:documentation>A type to document persistent identifiers.
Identifier values should be specified without a URI scheme (prefix),
instead the identifer type is documented as an attribute.
</xs:documentation>
4) Added maxOccurs="unbounded" to the usage in BaseNodeType to allow more than one Identifier:
<xs:element name="Identifier" type="fsx:IdentifierType" minOccurs="0" maxOccurs="unbounded"/>